Automatic gain control arrangements are used in amplifiers where it is desired to have a relatively constant output level for a wide range of input signals. As is well known in the art, such control is obtained by using negative feedback with variable elements so that the amplifier gain is a function of the values of the variable elements which values are controlled by the amplifier output level. If the output level is higher than desired, the variable element value in the feedback path is changed to reduce the amplifier gain. When the output level is lower than desired, the variable element value is modified to increase the amplifier gain. In this way, the amplifier output is held within a prescribed range over a much larger range of input signals.
Both resistive and capacitive elements have been used to obtain variable gain. Generally, resistive variations are obtained using a switched resistor or capacitor network in which the resistance or capacitance value is controlled by switching under digital control. U.S. Pat. No. 4,441,082 issued to Yusuf A. Haque, Apr. 3, 1984, for example, discloses a switched capacitor automatic gain control loop utilizing a capacitor array switched to vary the gain of an amplifier by an up-down counter responsive to the amplifier output level. The switching arrangement, however, may cause delay in the gain adjustment and noise effects. Elimination of such delay and noise can result in unduly complex circuitry.
A simpler arrangement employs a variable resistance semiconductor device such as a MOSFET controlled by the amplifier output level. In systems where there is more than one gain controlled amplifier and the gain control characteristics of the amplifiers must be coordinated, it is difficult to match the characteristics of the semiconductor resistance devices of the different amplifiers. U.S. Pat. No. 4,731,590 issued to Veikko R. Saari, Mar. 15, 1988, discloses a plurality of MOSFET variable resistance devices in a single signal control loop to provide gain control for a plurality of amplifier circuits. The automatic gain control curve and the dynamic range that can be accommodated, however, are limited by the characteristics of the MOSFET devices. It is an object of the invention to provide an improved gain control circuit adapted to coordinate the gain control of one or more amplifiers with selectable gain control characteristics over a large dynamic range.